1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to a Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication system that is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) long term evolution (LTE).
2. Description of the Art
Methods and apparatus are considered for the functionality and implementation of hopping for sequences used in the construction of Reference Signals (RS) or control signals transmitted in SC-FDMA communication systems.
The Uplink (UL) of the communication system is assumed, which corresponds to signal transmissions from mobile User Equipments (UEs) to a serving base station (Node B). A UE, also commonly referred to as terminal or mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a Base Transceiver System (BTS), an access point, or some other terminology. A Node B may control multiple cells in a cellular communication system, as it is known in the art.
Several types of signals need to be supported for the proper functionality of the communication system. In addition to data signals, which convey the information content of the communication, control signals also need to be transmitted from UEs to their serving Node B in the UL and from the serving Node B to UEs in the downlink (DL) of the communication system. The DL refers to the communication from the Node B to UEs. Additionally, a UE having data or control transmission also transmits RSs, also known as pilots. These RSs primarily serve to provide coherent demodulation for the transmitted data or control signals by a UE.
The UEs are assumed to transmit data or control signals over a Transmission Time Interval (TTI), which is assumed to correspond to a sub-frame. The sub-frame is the time unit of a frame, which may consist of ten sub-frames. FIG. 1 illustrates a block diagram of the sub-frame structure 110. The sub-frame 110 includes two slots. Each slot 120 further includes seven symbols used for transmission of data or control signals. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. The signal transmission in one slot may be in the same or at a different part of the operating bandwidth (BW) than the signal transmission in the second slot. In addition to symbols carrying data or control information, some symbols are used for Reference Signal (RS) transmission 140.
The transmission BW is assumed to include frequency resource units, which will be referred to herein as resource blocks (RBs). Each RB may consist of 12 sub-carriers and UEs are allocated a multiple N of consecutive RBs 150 for Physical Uplink Shared Channel (PUSCH) transmission and 1 RB for Physical Uplink Control Channel (PUCCH) transmission.
As the data or control signal transmission is over a BW that can be (orthogonally) shared by multiple UEs, the corresponding physical layer channel may be respectively referred to as PUSCH or as PUCCH. FIG. 1 illustrates a structure for the PUSCH sub-frame while respective ones for the PUCCH will be subsequently described.
The UEs are also assumed to transmit control signals in the absence of any data signals. The control signals include, but are not limited to, positive or negative acknowledgment signals (ACK or NAK, respectively) and Channel Quality Indication (CQI) signals. The ACK/NAK signals are in response to the correct or incorrect, respectively, data packet reception by a UE in the DL of the communication system. The CQI signals are sent by a UE to inform its serving Node B of its Signal-to-Interference and Noise Ratio (SINR) conditions in order for the serving Node B to perform channel dependent scheduling in the DL of the communication system. Both ACK/NAK and CQI signals are accompanied by RS signals in order to enable their coherent demodulation at the Node B receiver. The physical layer channel conveying ACK/NAK or CQI control signaling may be referred as the PUCCH.
The ACK/NAK, CQI and associated RS signals are assumed to be transmitted by UEs in one RB using CAZAC sequences as it is known in the art and is subsequently described.
FIG. 2 shows a structure for the ACK/NAK transmission during one slot 210 in a SC-FDMA communication system. The ACK/NAK information bits 220 modulate 230 a “Constant Amplitude Zero Auto-Correlation (CAZAC)” sequence 240, for example with QPSK or 16QAM modulation, which is then transmitted by the UE after performing an Inverse Fast Fourier Transform (IFFT) operation as it is further subsequently described. In addition to the ACK/NAK, RS is transmitted to enable the coherent demodulation of the ACK/NAK signal at the Node B receiver. The third, fourth, and fifth SC-FDMA symbols in each slot may carry an RS 250.
FIG. 3 shows a structure for the CQI transmission during one slot 310 in a SC-FDMA communication system. Similar to the ACK/NAK transmission, the CQI information bits 320 modulate 330 a CAZAC sequence 340, for example with QPSK or 16QAM modulation, which is then transmitted by the UE after performing the IFFT operation as it is further subsequently described. In addition to the CQI, RS is transmitted to enable the coherent demodulation at the Node B receiver of the CQI signal. In the embodiment, the second and sixth SC-FDMA symbols in each slot carry an RS 350.
As it was previously mentioned, the ACK/NAK, CQI, and RS signals are assumed to be constructed from CAZAC sequences. An example of such sequences is the Zadoff-Chu (ZC) sequences whose elements are given by Equation (1) below:
                                          c            k                    ⁡                      (            n            )                          =                              exp            ⁡                          [                                                                    j2π                    ⁢                                                                                  ⁢                    k                                    L                                ⁢                                  (                                      n                    +                                          n                      ⁢                                                                        n                          +                          1                                                2                                                                              )                                            ]                                .                                    (        1        )            L is the length of the CAZAC sequence, n is the index of an element of the sequence n={0, 1, 2 . . . , L−1}, and k is the index of the sequence itself. For a given length L, there are L−1 distinct sequences, if L is prime. Therefore, the entire family of sequences is defined as k ranges in {1, 2 . . . , L−1}. However, it should be noted that the CAZAC sequences used for the ACK/NAK, CQI, and RS transmission need not be generated using the exact above expression as it is further discussed below.
For CAZAC sequences of prime length L, the number of sequences is L−1. As the RBs are assumed to consist of an even number of sub-carriers, with 1 RB consisting of 12 sub-carriers, the sequences used to transmit the ACK/NAK, CQI, and RS can be generated, in the frequency or time domain, by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not fulfill the definition of a CAZAC sequence. Alternatively, the CAZAC sequences can be directly generated through a computer search for sequences satisfying the CAZAC properties.
A block diagram for the transmission through SC-FDMA signaling of a CAZAC-based sequence in the time domain is shown in FIG. 4. The selected CAZAC-based sequence 410 is generated through one of the previously described methods (modulated by the respective bits in case of ACK/NAK or CQI transmission), it is then cyclically shifted 420 as it is subsequently described, the Discrete Fourier Transform (DFT) of the resulting sequence is obtained 430, the sub-carriers 440 corresponding to the assigned transmission bandwidth are selected 450, the Inverse Fast Fourier Transform (IFFT) is performed 460, and finally the CP 470 and filtering 480 are applied to the transmitted signal 490. Zero padding is assumed to be performed by a UE in sub-carriers used for signal transmission by another UE and in guard sub-carriers (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas, as they are known in the art, are not shown in FIG. 4. Similarly, for the PUCCH, the modulation of a CAZAC sequence with ACK/NAK or CQI bits is well known in the art, such as for example QPSK modulation, and is omitted for brevity.
At the receiver, the inverse (complementary) transmitter functions are performed. This is conceptually illustrated in FIG. 5 where the reverse operations of those in FIG. 4 apply. As it is known in the art (not shown for brevity), an antenna receives the RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 510 passes through a time windowing unit 520 and the CP is removed 530. Subsequently, the receiver unit applies an FFT 540, selects 550 the sub-carriers 560 used by the transmitter, applies an Inverse DFT (IDFT) 570, de-multiplexes (in time) the RS and CQI signal 580, and after obtaining a channel estimate based on the RS (not shown) it extracts the CQI bits 590. As for the transmitter, well known receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
An alternative generation method for the transmitted CAZAC sequence is in the frequency domain. This is depicted in FIG. 6. The generation of the transmitted CAZAC sequence in the frequency domain follows the same steps as the one in the time domain with two exceptions. The frequency domain version of the CAZAC sequence is used 610 (that is the DFT of the CAZAC sequence is pre-computed and not included in the transmission chain) and the cyclic shift 650 is applied after the IFFT 640. The selection 620 of the sub-carriers 630 corresponding to the assigned transmission BW, and the application of CP 660 and filtering 670 to the transmitted signal 680, as well as other conventional functionalities (not shown), are as previously described for FIG. 4.
The reverse functions are again performed for the reception of the CAZAC-based sequence transmitted as in FIG. 6. This is illustrated in FIG. 7. The received signal 710 passes through a time windowing unit 720 and the CP is removed 730. Subsequently, the cyclic shift is restored 740, an FFT 750 is applied, and the transmitted sub-carriers 760 are selected 765. FIG. 7 also shows the subsequent correlation 770 with the replica 780 of the CAZAC-based sequence. Finally, the output 790 is obtained which can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of a RS, or can be used for detecting the transmitted information, in case the CAZAC-based sequence is modulated by ACK/NAK or CQI information bits.
The transmitted CAZAC-based sequence in FIG. 4 or FIG. 6 may not be modulated by any information (data or control) and can then serve as the RS, as shown, for example, in FIG. 2 and FIG. 3.
Different cyclic shifts of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different cyclic shifts of the same CAZAC sequence can be allocated to different UEs in the same RB for their RS or ACK/NAK, or CQI transmission and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 8.
Referring to FIG. 8, in order for the multiple CAZAC sequences 810, 830, 850, 870 generated correspondingly from multiple cyclic shifts 820, 840, 860, 880 of the same root CAZAC sequence to be orthogonal, the cyclic shift value Δ 890 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the duration of one symbol, the number of cyclic shifts is equal to the mathematical floor of the ratio Ts/D. For a CAZAC sequence of length 12, the number of possible cyclic shifts is 12 and for symbol duration of about 66 microseconds (14 symbols in a 1 millisecond sub-frame), the time separation of consecutive cyclic shifts is about 5.5 microseconds. Alternatively, to provide better protection against multipath propagation, only every other (6 of the 12) cyclic shift may be used providing time separation of about 11 microseconds.
CAZAC-based sequences of the same length typically have good cross-correlation properties (low cross-correlation values), which is important in order to minimize the impact of mutual interference in synchronous communication system and improve their reception performance. It is well known that ZC sequences of length L have optimal cross-correlation of √{square root over (L)}. However, this property does not hold when truncation or extension is applied to ZC sequences or when CAZAC-based sequences are generated through computer search. Moreover, CAZAC-based sequences of different lengths have a wide distribution of cross-correlation values and large values can frequently occur leading to increased interference.
FIG. 9 illustrates the Cumulative Density Function (CDF) of cross-correlation values for length-12 CAZAC-based sequence resulting from cyclically extending a length-11 ZC sequence, truncating a length-13 ZC sequence and generating length-12 CAZAC-based sequences through a computer search method. Variations in cross-correlation values can be easily observed. These variations have even wider distribution for cross-correlations between CAZAC-based sequences with different lengths.
The impact of large cross-correlations on the reception reliability of signals constructed from CAZAC-based sequences can be mitigated through sequence hopping. Pseudo-random hopping patterns are well known in the art and are used for a variety of applications. Any such generic pseudo-random hopping pattern can serve as a reference for sequence hopping. In this manner, the CAZAC-based sequence used between consecutive transmissions of ACK/NAK, CQI, or RS signals in different SC-FDMA symbols, can change in a pseudo-random pattern and this reduces the probability that CAZAC-based generated signals will be subjected to large mutual cross-correlations and correspondingly experience large interference over their transmission symbols.
There is therefore a need for supporting hopping of CAZAC-based sequences with minimum implementation complexity in order to reduce the average interference among CAZAC-based sequences.
There is another need for assigning CAZAC-based sequences through planning in different Node Bs and different cells of the same Node B in a communication system.
Finally, there is a need for minimizing the signaling overhead for communicating sequence hopping parameters or the sequence assignment (planning) from the serving Node B to the UEs.